1. Field of the Invention
The present invention relates generally to a mobile communication system for transmitting data using a plurality of transmission/reception antennas, and more particularly to an apparatus and method for accurately performing channel estimation at a plurality of reception antennas.
2. Description of the Related Art
Signals transmitted over a wireless channel are affected by a multi-path interference caused by various obstacles existing between a transmitter and a receiver. A wireless channel having multiple paths has a maximum delay spreading characteristic and a maximum signal transmission period characteristic. If the signal transmission period is longer than the maximum delay spreading time, there is no interference between successive signals, and a frequency area characteristic of a channel is set to a frequency nonselective fading. However, in a high-speed transmission system for transmitting data using a wideband signal, the signal transmission period is shorter than the maximum delay spreading time, such that signal interference between successive signals, and thus a received signal, is affected by intersymbol interference. In this case, the frequency area characteristic of the channel is set to a frequency selective fading. A single-carrier transmission method using a coherent modulation scheme utilizes an equalizer to remove such intersymbol interference. The higher the data transmission speed, the greater the signal distortion caused by the intersymbol interference. The greater the signal distortion, the higher the equalizer's complexity. An OFDM (Orthogonal Frequency Division Multiplexing) system has been recently proposed to solve the aforementioned problem of the equalizer for use in the single-carrier transmission method.
Typically, an OFDM scheme is defined as a two-dimensional access scheme for combining a TDM (Time Division Access) technique with an FDM (Frequency Division Access) technique. Therefore, when transmitting data using the OFDM scheme, individual OFDM symbols are separately loaded on sub-carriers, and are thus transmitted over predetermined sub-channels.
The OFDM scheme has superior spectrum efficiency because sub-channel spectrums are orthogonal to each other while being overlapped with each other. The OFDM scheme enables a modulation/demodulation unit to be implemented with an effective digital configuration because an OFDM modulation/demodulation is implemented with an IFFT (Inverse Fast Fourier Transform) and a FFT (Fast Fourier Transform). Further, the OFDM scheme is very well adapted to a current Europe digital broadcasting transmission and a high-speed data transmission prescribed in a high-capacity wireless communication system standard, for example, an IEEE 802.11a, an IEEE 802.16a, or an IEEE 802.16b, etc.
The OFDM scheme, serving as an MCM (Multi-Carrier Modulation), converts a serially-entered symbol stream into a parallel symbol stream, modulates the parallel symbol stream into a plurality of sub-carriers orthogonal to each other, and transmits the plurality of sub-carriers.
Such an MCM system was first applied to a high-frequency wireless communication for use in the military in the late 1950s, and an OFDM scheme for overlapping between a plurality of orthogonal sub-carriers was first studied in the late 1970s. This OFDM scheme must implement an orthogonal modulation between multiple carriers, resulting in limited system application. However, it was known that a modulation/demodulation based on the OFDM scheme can be effectively processed using a DFT (Discrete Fourier Transform), and many developers have conducted intensive research into the OFDM scheme. Using a guard interval and a method for inserting a cyclic prefix guard interval are well known to those skilled in the art, such that negative influence on a system affected by a multiple-path and a delay spread is greatly reduced. Therefore, the OFDM scheme is widely applied to digital transmission technology, for example, DAB (Digital Audio Broadcasting), digital TV, a W-LAN (Wireless-Local Area Network), and a W-ATM (Wireless Asynchronous Transfer Mode). More specifically, although the use of the OFDM scheme has been limited due to its hardware complexity, the OFDM scheme can be implemented with digital signal processing technology such as a FFT and an IFFT. The OFDM scheme is similar to a conventional FDM (Frequency Division Multiplexing) scheme, but it can obtain optimum transmission efficiency during high-speed data transmission because it transmits a plurality of sub-carriers orthogonal to each other. Further, the OFDM scheme has superior frequency use efficiency and is very resistive to a multi-path fading, which results in an optimum transmission efficiency during high-speed data transmission. Particularly, because the OFDM scheme uses an overlapped frequency spectrum, it can effectively use a frequency, is very resistive to a frequency selective fading and a multi-path fading, reduces an intersymbol interference using a guard interval, and provides an equalizer composed of simple hardware. Also, the OFDM scheme is very resistive to an impulse noise, such that it is widely used in communication system architecture.
FIG. 1 is a block diagram of a conventional mobile communication system based on an OFDM scheme. Referring to FIG. 1, an input bit, which is a binary signal, is applied to a channel encoder 100. The channel encoder 100 codes input bits, and outputs coded symbols. The coded symbols are applied to a S/P (Serial/Parallel) converter 105. The S/P converter 105 converts received serial-coded symbols into parallel-coded symbols, and transmits the parallel-coded symbols to a modulator 110. The modulator 110 maps the received coded symbols with a symbol-mapping constellation, and outputs mapped symbols. There are a variety of modulation schemes for use in the modulator 110, for example, a QPSK, a 8PSK, a 16QAM, a 64QAM, etc. The number of bits contained in the symbols is prescribed according to individual modulation schemes. The QPSK modulation scheme is composed of 2 bits, the 8PSK modulation scheme is composed of 3 bits, the 16QAM modulation scheme is composed of 4 bits, and the 64QAM modulation scheme is composed of 6 bits. Modulated symbols generated from the modulator 110 are applied to an IFFT unit 115. The IFFT-modulated symbols generated from the IFFT unit 115 are applied to a P/S (Parallel/Serial) converter 120, and the P/S converter 120 outputs serial-format symbols. The serial-format symbols are transmitted over a transmission antenna 125.
The symbols transmitted from the transmission antenna 125 are received at a reception antenna 130. Symbols received at the reception antenna 130 are converted into parallel-format symbols over the S/P converter 135. The parallel-format symbols are transmitted to a FFT unit 140. A reception signal received at the FFT unit 130 performs an FFT process, and is then applied to a demodulator 145. The demodulator 145 has a symbol mapping constellation to the same as that of the modulator 110, and converts despreading symbols into binary-bit symbols according to the symbol mapping constellation. That is, the demodulation scheme is determined by the modulation scheme. Binary symbols demodulated by the demodulator 145 are used for channel estimation by a channel estimator 150. The channel estimator 150 estimates various conditions created when the transmission antenna 125 transmits data, resulting in an effective data reception. The binary symbols for performing channel estimation using the channel estimator 150 are converted into serial-format symbols over the P/S converter 155, and are then decoded by a decoder 160. The binary symbols applied to the channel decoder 160 are decoded and thus binary bits are generated from the channel decoder 160.
FIG. 2 is a block diagram of a mobile communication system for transmitting/receiving data according to an OFDM scheme using a plurality of transmission/reception antennas. However, prior to describing the above mobile communication system, a mobile communication system for transmitting/receiving data using one transmission/reception antenna will hereinafter be described in more detail.y(n)=x(n)h(n)+n(n)  [Equation 1]
With reference to the Equation 1, y(n) is data received at a time “N” over a reception antenna, x(n) is data transmitted at a time “N” over a transmission antenna, h(n) is an index of a transmission channel environment influence of data generated from the transmission antenna at a specific time “N”, and n(n) is a noise created at the time “N”. The influence of the noise will herein be omitted for the convenience of description of the present invention.
As shown in the Equation 1, a reception end must previously know the value of h(n) to obtain correct data. For this purpose, a transmission/reception end of a mobile communication system transmits previously-known data in such a way that the value of h(n) can be recognized. This previously-known data is called a training symbol. Provided that the value of h(n) is recognized, a reception end of the mobile communication system can correctly recognize data transmitted from the transmission end.
As illustrated in FIG. 2, each of a plurality of transmission antennas 220, 222, and 224 transmits data using a plurality of sub-carriers having a specific frequency. The plurality of sub-carriers are assigned to the plurality of transmission antennas 220, 222, and 224 in the OFDM-based mobile communication system using a plurality of transmission/reception antennas.
Modulators 200, 202, and 204 modulate received symbols, and transmit the modulated symbols to the IFFT units 210, 212, and 214. The IFFT units 210, 212, and 214 IFFT-modulate the received symbols, and transmit IFFT-modulated symbols over individual transmission antennas 220, 222, and 224. Data transmitted over the transmission antennas 220, 222, and 224 is received at reception antennas 230, 232, and 234. The data received at the reception antennas 230, 232, and 234 is FFT-modulated at FFT units 240, 242, and 244, and is then transmitted to demodulators 250, 252, and 254. Channel estimators 260, 262, and 264 perform channel estimation on symbols demodulated at the demodulators 250, 252, and 254.
Individual reception antennas 230, 232, and 234 receive data transmitted from each of the transmission antennas 220, 222, and 224. In more detail, the reception antenna 230 receives data transmitted from the transmission antennas 220, 222, and 224, the reception antenna 232 receives data transmitted from the transmission antennas 220, 222, and 224, and the reception antenna 234 receives data transmitted from the transmission antennas 220, 222, and 224.
Individual sub-carriers having a specific frequency allocated to the transmission antennas are allocated to different transmission antennas. If the number of allocatable sub-carriers is “A” and the number of transmission antennas is “B”; an A/B number of sub-carriers is generally allocated to one transmission antenna for transmission of the training symbol.xp(n)=[0 . . . 0x1px2p . . . xNap0xNa+1p. . . x2Nap0 . . . 0]T  [Equation 2]
Sub-carriers allocated to an OFDM-based mobile communication system are represented in the Equation 2. With reference to the Equation 2, xp(n) is a training symbol transmitted at a N-th time from a P-th antenna in the OFDM-based system having K carriers. The training symbol is a symbol recognized at a transmission/reception end of the system to perform channel estimation, is loaded on a sub-carrier having a specific frequency, and is then transmitted. As shown in the Equation 2, all the sub-carriers are not allocatable, but only some sub-carriers from among all sub-carriers are allocatable. A center carrier and both-ends carriers having a DC component are not allocated to the transmission antennas 220, 222, and 224. Therefore, the number of sub-carriers allocatable to the training symbol is 2Na. Provided that the number of transmission antennas is Nt, sub-carriers for transmission of Nc training symbols are allocated to one transmission antenna.2Na=NcNt  [Equation 3]
Therefore, training symbols are allocated to individual transmission antennas, and the allocated training symbols are transmitted over sub-carriers.
                                                                        x                i                p                            =                              {                                                                                                    c                        i                                                                                                            i                        =                                                                                                            (                                                              m                                -                                1                                                            )                                                        ⁢                                                          N                              t                                                                                +                          p                                                                                                                                                0                                                              otherwise                                                                                                                                                              0                ≤                p                ≤                                                      N                    t                                    -                  1                                            ,                              1                ≤                i                ≤                                                      N                    c                                    ⁢                                      N                    t                                                                                                          [                  Equation          ⁢                                          ⁢          4                ]            
where xip is a training symbol included in the pth training symbol group, Nt is the number of antennas or the number of training symbol groups, ci is an arbitrary complex of a magnitude ∞{square root over (Nt)}, m is an integer lower than Nc, and Nc is number of training symbols allocated to one transmission antenna.
When three transmission antennas are applied to the Equation 4, FIG. 3 is a view illustrating training symbols transmitted over these three transmission antennas. Referring to FIG. 3, individual transmission antennas do not transmit data in a virtual carrier area and a DC carrier area. Individual transmission antennas load the training symbols on the sub-carriers of a specific frequency according to the Equation 4, and then transmit the training symbols loaded on the sub-carriers. If the number of the training symbols having a specific frequency is 12, a first transmission antenna transmits a first training symbol, a fourth training symbol, a seventh training symbol, and a tenth-training symbol. A second transmission antenna transmits a second training symbol, a fifth training symbol, an eighth training symbol, and an eleventh training symbol. A third transmission antenna transmits a third training symbol, a sixth training symbol, a ninth training symbol, and a 12-th training symbol. A group of the training symbols transmitted at a specific time over individual transmission antennas is called a training symbol group. The training symbol group is applied to individual modulators illustrated in FIG. 2, but it will hereinafter be described in connection with a transmission antenna.
The training symbols transmitted over several transmission antennas are received at a plurality of reception antennas over individual transmission paths (i.e., channels). Individual reception antennas receive training symbols received from the transmission antennas. Therefore, one reception antenna must estimate status information of a channel for transmitting the training symbols over the transmission antennas.
TABLE 1TransmissionTransmissionTransmissionTransmissionantenna 1antenna 2antenna 3antenna NReceptionh11h21h31hN1antenna 1Receptionh12h22h32hN2antenna 2Receptionh13h23h33hN3antenna 3Receptionh1Mh2Mh3MhNMantenna M
Table 1 indicates channel estimation values to be measured at individual reception antennas for receiving training symbols from transmission antennas. The value of h11 is a channel estimation value measured at a reception antenna 1 using a training symbol received from the transmission antenna 1. The value of hNM is a channel estimation value measured at a reception antenna M using a training symbol received from the transmission antenna N. Various values created at specific time points are shown in the Table 1. The channel estimation values may be denoted by a two-dimensional matrix. The channel estimation values denoted by such a two-dimensional matrix at a specific time are called a spatial channel matrix. As shown in the Table 1, one reception antenna must measure channel estimation values associated with a plurality of transmission antennas. The channel estimation values measured by a specific reception antenna measure the training symbols several times to reduce the influence of noises. In this case, individual transmission antennas transmit a training symbol group composed of the same training symbols to the reception antennas.
TABLE 2Trans-missionTransmissionTransmissionTransmissionantenna 1antenna 2antenna 3antenna NT1TrainingTraining symbolTraining symbolTraining symbolsymbolgroup 2group 3group Ngroup 1T2TrainingTraining symbolTraining symbolTraining symbolsymbolgroup 2group 3group Ngroup 1T3TrainingTraining symbolTraining symbolTraining symbolsymbolgroup 2group 3group Ngroup 1T4TrainingTraining symbolTraining symbolTraining symbolsymbolgroup 2group 3group Ngroup 1
With reference to the Table 2 above, the specific transmission antenna transmits the same training symbol group to the reception antennas to measure channel estimation values. It should be noted that the training symbol group is not one training symbol, but is composed of a plurality of training symbols having a specific frequency according to Equation 4.
Individual reception antennas receive training symbol groups transmitted from transmission antennas, and perform channel estimation on the basis of the values of the received training symbols. The reception antennas repeatedly perform the channel estimation. A channel estimation value correctly measured at the reception antennas is obtained from only a specific-frequency carrier having a training symbol loaded from the transmission antennas. Other channel estimation values of sub-carriers associated with a specific frequency unloaded from the transmission antenna cannot be recognized at the reception antennas.
In a system having three transmission antennas and 12 sub-carriers, a first transmission antenna from among the three transmission antennas loads a training symbol on a first sub-carrier, a fourth sub-carrier, a seventh sub-carrier, and a tenth sub-carrier. The reception antenna receives the first sub-carrier, the fourth sub-carrier, the seventh sub-carrier, and the tenth sub-carrier from the first transmission antenna, and measures channel estimation values of individual channel paths using the received sub-carriers. However, it is impossible for the reception antenna to perform channel estimation on a second sub-carrier, a third sub-carrier, a fifth sub-carrier, a sixth sub-carrier, an eighth sub-carrier, a ninth sub-carrier, an 11-th sub-carrier, and a 12-th sub-carrier.
In order to solve this problem, recently an interpolation method for estimating channels of sub-carriers having no training symbol using channel estimation values of other sub-carriers having training symbols has been proposed. However, in this case, a training symbol is not allocated to an edge sub-carrier area, which results in an ineffective interpolation method and an increased channel estimation error. FIGS. 4A˜4B illustrate channel estimation results using a conventional interpolation method. Referring to FIGS. 4A˜4B, training symbols are transmitted over four transmission antennas, and channel estimation error increases at both-edge sub-carrier areas. Therefore, a need exists for a method for performing accurate channel estimation even in a sub-carrier area having no training symbol.